TEM study of a special grain boundary in a synthetic K-feldspar bicrystal: Manebach Twin

 A monoclinic KAlSi₃O₈ feldspar Manebach twin boundary was synthesized by diffusion bonding and examined using high-resolution transmission electron microscopy. The sharp (001) twin boundary is straight and free of strain. The boundary width is smaller than d₀₀₁. There is no rigid body shift observed at the twin boundary, and the feldspar structure is arranged symmetrically across (001). The twin boundary structure consists of bridged tetrahedral crankshafts, which are characteristic of the feldspar lattice. The grain boundary structure is in good agreement with the geometrical model of Taylor et al. (1934). The grain boundary composition of K_1/2H_1/2AlSi₃O₈ differs from their model. Received: 13 February 2002 / Accepted: 24 December 2002 Acknowledgements We thank M. Rühle, S. Hutt, J. Mayer, A. Strecker and U. Salzberger at MPI, Stuttgart, for their support and valuable advice in preparing TEM sections of bicrystals.     

Estimation of causative fault parameters of the Rudbar earthquake of June 20, 1990 from near field SH-wave data

We analyze the strong motion accelerograms recorded for the large (M_S=7.7, M_W=7.3, m_b=6.4) Rudbar earthquake of June 20, 1990. The earthquake had a complex source process. We have identified the imprints of rupture of three localized asperities on the major causative fault on the accelerograms. These asperities are interpreted to correspond to (i) the main shock that initiated the rupture process and was located in the domino block between the Kabateh and Zard Goli faults, (ii) a foreshock that occurred about 10 s earlier in the Kabateh fault and (iii) a later shock, on the western end of the Baklor fault, which terminated the bilateral rupture process at the western end. We estimate the strike, dip and slip of these causative sub-event rupture planes using the SH spectral amplitudes, based on a point source representation of sub-events and a non-linear least square formulation for inversion of the amplitude data. The results of our inversion of the near field data are comparable to other studies based on teleseismic data.     

Early Cretaceous planktonic foraminifera from the Tethys: the large, many-chambered representatives of the genus Globigerinelloides

This paper deals with the taxonomic revision of the Early Cretaceous large, many-chambered planispiral planktonic foraminifera, historically assigned to the genus Globigerinelloides or alternatively assigned in the 1990s to the genera Globigerinelloides Cushman and ten Dam, Biglobigerinella Lalicker, Blowiella Krechmar and Gorbachik and Alanlordella BouDagher-Fadel. In a previous paper we demonstrated that the morphological and microstructural features used in the literature for distinguishing Blowiella from Globigerinelloides have value only at species level, and the former genus was thus invalidated (being the junior synonym). Moreover, the Late Aptian specimens assigned to Biglobigerinella by some authors, based on the presence of twin last chamber(s), are also included in Globigerinelloides because individuals sharing the same features (number of chambers, growth rate, size of umbilicus, and a finely perforate wall) may or may not possess twin last chamber(s). Meanwhile, Moullade et al. questioned the taxonomic value of Alanlordella, erected by BouDagher-Fadel to accommodate planispiral taxa possessing a macroperforate wall. All the species analysed here possess a finely perforate wall and consequently cannot be assigned to this taxon.The large species of Globigerinelloides retained here, with six or more chambers in the outer whorl, are G. algerianus Cushman and ten Dam, G. aptiensis Longoria, G. barri (Bolli, Loeblich and Tappan) and G. ferreolensis (Moullade).In the sections studied, Globigerinelloides aptiensis was first found close to the Barremian/Aptian boundary, even though this species was recorded in Spain (Rio Argos) in the mid Upper Barremian; very rare, small, seven-chambered individuals here assigned to Globigerinelloides ferreolensis are recorded in the Lower Aptian (just below and within the Selli Level, OAE1a), while a few specimens belonging to Globigerinelloides barri occur in the Globigerinelloides ferreolensis Zone (Upper Aptian). Globigerinelloides aptiensis and G. ferreolensis range up to the Ticinella bejaouaensis Zone while Globigerinelloides barri disappears at the top of the Globigerinelloides algerianus Zone; finally, Globigerinelloides algerianus obviously spans the eponymous total range zone.From an evolutionary point of view, two lineages within the many-chambered Globigerinelloides have been recognized. In the first, already known in the literature, Globigerinelloides aptiensis gave rise to G. ferreolensis, which evolved into G. algerianus; the latter in turn gave rise to Pseudoplanomalina cheniourensis as the final evolutionary member. In the second lineage Globigerinelloides barri originated from G. blowi.     

A millennial-scale record of Holocene tsunamis on the Kronotskiy Bay coast, Kamchatka, Russia

Deposits from as many as 50 large tsunamis during the last 7000 years are preserved on the Pacific coast of the Kamchatka Peninsula near the mouth of the Zhupanova River, southern Kronotskiy Bay. These deposits are dated and correlated using Holocene marker tephra layers. The combined, preserved record of tsunami deposits and of numerous marker tephras on Kamchatka offers an unprecedented opportunity to study tsunami frequency. For example, from the stratigraphy along southern Kronotskiy Bay, we estimate frequency of large tsunamis (>5 m runup). In the last 3000 years, the minimum frequency is about one large tsunami per 100 years, and the maximum about one large tsunami per 30 years; the latter frequency occurred from about 0 to 1000 A.D. This time interval corresponds to a period of increased seismicity and volcanic activity that appears to be recorded in many places on the Kamchatka Peninsula.     

Predators of juvenile blue crabs outside of refuge habitats in lower Chesapeake Bay

Blue crabsCallinectes sapidus in lower Chesapeake Bay are subject to high rates of predation during the late summer of their first year of growth as they migrate out of vegetated nursery habitats. Predators, potentially contributing to this pattern, were identified in video-recorded field observations of tethered juvenile crabs (20–25 mm carapace width). Predators were also tested in large laboratory tanks containing similarly-sized untethered crabs as prey. Seven different predators attacked tethered crabs in the field. Only two predators, larger blue crabs and northern puffers,Sphoeroides maculatus, consistently succeeded in preying on crabs in both field and laboratory settings. These results confirm the importance of cannibalism on juvenile blue crabs and identify puffers as a potentially overlooked source of predation pressure.     

Metamorphic and deformational imprint of Cambrian–Lower Ordovician rifting in the Ossa-Morena Zone (Iberian Massif, Spain)

The high-temperature metamorphism recorded in the Valuengo and Monesterio areas constitutes a rare occurrence in the Ossa-Morena Zone of Southwest Iberia, where low-grade metamorphism dominates. The metamorphism of the Valuengo area has been previously considered either Cadomian or Variscan in age, whereas that of Monesterio has been interpreted as a Cadomian imprint. However, these areas share important metamorphic and structural features that point towards a common tectonometamorphic evolution. The metamorphism of the Valuengo and Monesterio areas affects Late Proterozoic and Early Cambrian rocks, and is syn-kinematic with a top-to-the-north mylonitic foliation, which was subsequently deformed by early Variscan folds and thrusts. The U–Pb zircon age (480±7 Ma) we have obtained for an undeformed granite of the Valuengo area is consistent with our geological observations constraining the age of the metamorphism. We propose that this high-temperature metamorphic imprint along a NW–SE ductile extensional shear zone is related to the crustal extension that occurred in the Ossa-Morena Zone during the Cambro-Ordovician rifting. In the same way, the tectonothermal effect of the preorogenic rifting stage may have been wrongly attributed to orogenic processes in other regions as well as in this one.     

Computational study of tetrahedral Al–Si and octahedral Al–Mg ordering in phengite

 As part of a wider study of the nature and origins of cation order–disorder in micas, a variety of computational techniques have been used to investigate the nature of tetrahedral and octahedral ordering in phengite, K₂ ^[6](Al₃Mg)^[4](Si₇Al)O₂₀(OH)₄. Values of the atomic exchange interaction parameters J _n used to model the energies of order–disorder were calculated. Both tetrahedral Al–Si and octahedral Al–Mg ordering were studied and hence three types of interaction parameter were necessary: for T–T, O–O and T–O interactions (where T denotes tetrahedral sites and O denotes octahedral sites). Values for the T–T and O–O interactions were taken from results on other systems, whilst we calculated new values for the T–O interactions. We have demonstrated that modelling the octahedral and tetrahedral sheets alone and independently produces different results from modelling a whole T–O–T layer, hence justifying the inclusion of the T–O interactions. Simulations of a whole T–O–T layer of phengite indicated the presence of short-range order, but no long-range order was observed. Received: 8 August 2002 / Accepted: 14 February 2003 Acknowledgements The authors are grateful to EPSRC (EJP) and the Royal Society (CIS) for financial support. Monte Carlo simulations were performed on the Mineral Physics Group's Beowulf cluster and the University of Cambridge's High Performance Computing Facility.     

Raman scattering and X-ray diffraction study of the thermal decomposition of an ettringite-group crystal

 A Raman scattering and X-ray diffraction study of the thermal decomposition of a naturally occurring, ettringite-group crystal is presented. Raman spectra, recorded with increasing temperature, indicate that the thermal decomposition begins at ≈55 °C, accompanied by dehydration of water molecules from the mineral. This is in contrast to previous studies that reported higher temperature breakdown of ettringite. The dehydration is completed by 175 °C and this results in total collapse of the crystalline structure and the material becomes amorphous. The Raman scattering results are supported by X-ray diffraction results obtained at increasing temperatures. Received: 9 July 2001 / Accepted: 14 August 2002     

The Upper Neoproterozoic-Lower Cambrian of the Central Iberian Zone, Spain: chemical and isotopic (Sm-Nd) evidence that the sedimentary succession records an inverted stratigraphy of its source

The Upper Neoproterozoic-Lower Cambrian sedimentary succession in the Central Iberian Zone has recently been divided into 12 sedimentary units (I to XII). Units I to IV are of Late Neoproterozoic age, and units V to XII are Early Cambrian.Throughout the stratigraphic section, shales with similar ranges of SiO₂ and Al₂O₃ have TiO₂ and Zr contents that are coupled and vary gradually from the bottom (unit I, TiO₂ = 1.0-1.1%, Zr = 245-287 ppm) to the top (unit XII, TiO₂ = 0.74-0.86%, Zr = 141-192 ppm). These two parameters clearly distinguish Upper Neoproterozoic from Lower Cambrian compositions, as do certain elemental ratios (Al₂O₃/TiO₂, Rb/Zr, Nb/Ti), which vary from low values at the bottom of the section to high at the top. However, other elements (the rare earth elements [REEs], Y) do not fit this general trend, with heavy REEs and Y showing extreme concentrations in some Lower Cambrian units, which also have negative Ce anomalies.Units I to III and units XI to XII have uniform T_DM neodymium model ages (∼1.1 and 1.7 Ga, respectively), but the units between these two groups have erratic values between 1.4 Ga (unit VII) and 3.4 Ga (unit VIII). Thus, the bottom and top units have neodymium isotope ratios that indicate the presence of two source compositions: a composition that records a juvenile contribution younger than 1.1 Ga and a composition mainly derived from an old basement. Some samples from Lower Cambrian units have disturbed REE abundances and neodymium isotope ratios that are attributed to extensive chemical alteration that resulted in REE and Y redistribution. This alteration probably affected the material now preserved as unit IV during a period of relative sea level fall at the end of the Late Neoproterozoic. Both elemental and isotopic results support the suggestion that the Upper Neoproterozoic-Lower Cambrian stratigraphic section records an inverse stratigraphy of a source hinterland composed of a cover sequence of relatively juvenile crustal materials underlain by an older basement. Through time, the deeper crust gradually increased its contribution to the sediments in the basin from Late Neoproterozoic to Lower Cambrian times. These results are compatible with data from many other European zones.